The invention relates to a method for scanning optical interference patterns with line sensors according to the preamble of the main claim. In particular, a description is given of a method for scanning optical interference patterns arising on superimposing two time partly coherent, non-parallel light beams on a linear image sensor.
An example of a linear image sensor is a line sensor comprising linear juxtaposed, light-sensitive pixels, which can be electronically read in order to detect an interference pattern with a measuring device, e.g. a PC. A linear image sensor can also comprise several juxtaposed pixel lines. In exemplified manner hereinafter reference is simply made to line sensors.
Such scanning processes for optical interference signals are of a standard nature in optical coherence tomography (OCT), particularly when carried out without movable components, as is e.g. described in WO 2002/084263 A1. It is also known that the scanning depth of such a “NoMotion-OCT” a priori is very limited compared with other OCT methods, where scanning depths of a few millimeters are obtained.
In order to obtain a comparable scanning depth, it is e.g. possible to place a so-called stepped mirror, which reflects components of the reference beam with different path lengths, in place of a mirror in the reference arm of a Michelson interferometer. Such a mirror, which must have step heights in the micrometer range, is far from easy to manufacture.
The solution of DE 196 15 616 A1 is simpler in practice and therein the reference arm mirror is replaced by a tilted reflection grating. This leads to reflections in the directions of the diffraction orders of the grating and one of said reflected beams is deflected onto the detector. Using beam optics (lens), placed in the diffraction pattern of the grating, in the solid angle range of a selected diffraction order, the individual grooves are imaged on different pixels of the detector in order to obtain a pixel-ordered transit time distribution of the reference light beam. Thus, the detector is no longer in the diffraction pattern and in particular imaging cancels out the divergence of different spectral components by grating diffraction on the detector. According to DE 196 15 616 A1, the grating is only partly in the focal plane of the beam optics, so that imaging is not equally successful on all the pixels, which gives rise to additional evaluation problems.
In the test assembly of U.S. Pat. No. 5,943,133 said problem is obviated, because the reflection grating, focal plane and detector are precisely parallel. The measurement and reference light beams are diffracted by means of the same grating in such a way that reflections of both beams impinge on the detector substantially parallel with different diffraction orders. Here again an image and not the diffraction pattern of the grating is detected on the detector. The imaging of the grating on the detector is used for the practical implementation of a stepped mirror from which is obtained a clearly defined transit time distribution on the detector pixels. Through the simultaneous reflection of measurement and reference light said virtual stepped mirror has twice the step height compared with DE 196 15 616 A1.
However, the precise imaging of the finally structured grating (structural sizes of a few millimeters have to be resolved) makes high demands on the beam optics and account must also be taken in the evaluation of aberrations as a further error source.
In other test assemblies, such as e.g. according to WO 2002/084263 A1, there is no need for imaging optics. Normally here there is only a beam focussing perpendicular to the sensor line, e.g. with a cylindrical lens, in order to obtain an intensity rise for the measurement light on the detector. This gives rise to no evaluation problems.
In the structure of WO 2002/084263 A1 the scanning depth can be particularly easily increased, in that on superimposing on the line sensor the reference beam can be tilted against the specimen beam. However, this leads to a finer spatial structure of the amount of the electrical field and therefore the light intensity distribution directly at the detector. Thus, there are far more interference fringes on the same detector surface. Normally there are more interference fringes per pixel. However, for scanning a sine wave train the scanning theorem requires at least two scans per full wave. The underscanning of the interference signal is very unfavourable with line sensors, because they can only measure in integrating manner over pixel surfaces, so that it is not readily possible to reconstruct an underscanned signal. Underscanning must be avoided for appropriate evaluation.
Hitherto there has been no technically readily practicable solution of the scanning problem with respect to a sensor having a higher pixel density (approximately 10,000 pixels), because it is expensive to manufacture and can only be read with difficulty. Standard line sensors have approximately 1,000 pixels.
DE 10 2004 033 187 B3 discloses an easy way out for the case where interest is only attached to the mean course of an intensity amplitude distribution as opposed to the detection of the complete interference pattern. This is the true measurement function of the OCT. The optical interference signal appears as an amplitude-modulated, rapidly oscillating intensity distribution along the sensor line and the specimen information is carried not by the interference fringes, but their envelope. The term oscillation here means a time-stationary carrier frequency measured as the reciprocal length on the image sensor.
In the case of the OCT, where short-coherent light is used, the envelope is a convolution of the coherence function with an interference signal, which arises through transit time distribution in the specimen. The coherence function is determined once and for all for a light source and then it is possible to calculate the transit time distribution from the envelope. Thus, DE 10 2004 033 187 B3 proposes to avoid the underscanning of the interference pattern by a suitable masking of the line sensor. The periodic mask to be used multiplies the interference signal in such a way that slowly oscillating components arise, which can be readily scanned with the given pixel resolution. Using telecommunications engineering language, mixing takes place on a low frequency intermediate band. However, the disadvantage of this measure is that considerable components of the already weak useful light with specimen information scattered back by the specimen are blocked out by the mixing process.
It is assumed hereinafter that on the line sensor is obtained an interference pattern by superimposing two time partly coherent, non-parallel, incident light beams, which can not be completely scanned according to the Nyquist condition with the given sensor pixel density. The interference pattern is essentially characterized by a carrier frequency (expressed as the number of interference fringes per pixel) and an amplitude modulation. It is assumed that the amplitude modulation is more slowly variable compared with the carrier wave, particularly approximately constant over a single pixel width. The latter is fundamentally not a restriction, because this is the case with any practical test assembly. It is left open as to the significance of amplitude modulation in each individual case.